xAs Buffer Layers Research Articles

Numerical simulation of graphene/GaAs heterojunction solar cells

For a thorough understanding of graphene/GaAs heterojunction solar cells, the effect of the graphene work function, absorption loss, density of interface states, as well as the incorporation of an AlxGa1−xAs (0≤x≤1) buffer layer on the performance of graphene/GaAs heterojunction solar cells was investigated via AFORS-HET simulation software. It is demonstrated that graphene with a work function of approximately 5 eV favors the efficiency improvement. The absorption loss is also an unnegligible factor for the optimization of the graphene work function. The density of interface states can significantly affect the carrier recombination at the heterojunction interface between graphene and GaAs and the corresponding photovoltaic performance. In addition, the work function of a back electrode should also be taken into account for selecting a back electrode material due to its significant influence on the carrier transportation and collection. Furthermore, when an AlxGa1−xAs (x = 0.2) buffer layer was introduced to this heterojunction interface, an enhanced photovoltaic conversion efficiency of 24.78% can be achieved. Finally, a comparison among simulation and experimental data for the graphene/GaAs heterojunction solar cells is presented. This study indicates the great potential of graphene/GaAs heterojunction for the application in high-efficiency solar cells.

Studying Magnetic Diodes with a GaMnAs Layer Formed by Pulsed Laser Deposition

A new design for diode heterostructures with (Ga, Mn)As ferromagnetic layers is experimentally investigated. The diode structures are fabricated using a combination of metal-organic chemical vapor deposition (MOCVD) epitaxy and pulsed laser deposition and contain (Ga, Mn)As/n-InGaAs heterojunctions. The electrical properties of the diodes in a magnetic field applied perpendicular to the p–n junction are examined. The negative magnetoresistance observed at temperatures of <50 K (below the (Ga, Mn)As Curie temperature) is attributed to a decrease in the carrier scattering caused by ferromagnetic ordering and in the potential barrier at the (Ga, Mn)As/n-InGaAs interface. The observed negative magnetoresistance depends nonmonotonically on the forward bias with a maximum in the voltage region close to the p–n-junction potential barrier height. The maximum can be caused by the decisive contribution of the spin-dependent resistances of the (Ga, Mn)As layer and (Ga, Mn)As/n-InGaAs interface to the total resistance of the structure. It is found that the dependence of the magnetoresistance on the external magnetic field is hysteretic due to the influence of tensile stresses in the (Ga, Mn)As layer grown on top of a relaxed InxGa1 –xAs buffer layer with an indium content of x ≈ 0.1.

Effect of Strongly Mismatched GaAs and InAs Inserts in a InAlAs Buffer Layer on the Structural and Optical Properties of Metamorphic InAs(Sb)/InGaAs/InAlAs/GaAs Quantum-Confined Heterostructures

Metamorphic InAs(Sb)/InGaAs/InAlAs quantum-confined heterostructures with thin (1–5 nm) strongly mismatched GaAs and InAs inserts in a gradient metamorphic InxAl1−xAs buffer layer have been grown on GaAs (001) substrates by molecular beam epitaxy. It has been shown that the use of a 5-nm GaAs insert in the region of a metamorphic buffer layer at x ~ 0.37 almost doubles the photoluminescence intensity at 300 K (λ ~ 3.5 μm) from an InAs/InGaAs quantum well with a monolayer InSb insert. This is explained by an increase in the hole localization energy in InSb measured by photomodulation reflection Fourier transform infrared spectroscopy. This increase is due to increased elastic stresses in the quantum well because of a decreased density of threading dislocations in this structure caused by the introduction of an additional inverse step into the metamorphic buffer layer in the form of a 5-nm GaAs layer. The introduction of a 5-nm InAs layer into the metamorphic buffer layer violates its functions as a dislocation filter, leading to a higher density of dislocations in the quantum well region and to an order of magnitude decrease in the luminescence intensity.

The Effect of Buffer Types on the In0.82Ga0.18As Epitaxial Layer Grown on an InP (100) Substrate.

In0.82Ga0.18As epitaxial layers were grown on InP (100) substrates at 530 °C by a low-pressure metalorganic chemical vapor deposition (LP-MOCVD) technique. The effects of different buffer structures, such as a single buffer layer, compositionally graded buffer layers, and superlattice buffer layers, on the crystalline quality and property were investigated. Double-crystal X-ray diffraction (DC-XRD) measurement, Raman scattering spectrum, and Hall measurements were used to evaluate the crystalline quality and electrical property. Scanning electron microscope (SEM), atomic force microscope (AFM), and transmission electron microscope (TEM) were used to characterize the surface morphology and microstructure, respectively. Compared with the In0.82Ga0.18As epitaxial layer directly grown on an InP substrate, the quality of the sample is not obviously improved by using a single In0.82Ga0.18As buffer layer. By introducing the graded InxGa1−xAs buffer layers, it was found that the dislocation density in the epitaxial layer significantly decreased and the surface quality improved remarkably. In addition, the number of dislocations in the epitaxial layer greatly decreased under the combined action of multi-potential wells and potential barriers by the introduction of a In0.82Ga0.18As/In0.82Al0.18As superlattice buffer. However, the surface subsequently roughened, which may be explained by surface undulation.

Growth temperature optimization of GaAs-based In0.83Ga0.17As on InxAl1−xAs buffers

Improved quality of gas source molecular beam epitaxy grown In0.83Ga0.17As layer on GaAs substrate was achieved by adopting a two-step InxAl1−xAs metamorphic buffer at different temperatures. With a high-temperature In0.83Al0.17As template following a low-temperature composition continuously graded InxAl1−xAs (x = 0.05–0.86) buffer, better structural, optical and electrical properties of succeeding In0.83Ga0.17As were confirmed by atomic force microscopy, photoluminescence and Hall-effect measurements. Cross-sectional transmission electron microscopy revealed significant effect of the two-step temperature grown InAlAs buffer layers on the inhibition of threading dislocations due to the deposition of high density nuclei on GaAs substrate at the low growth temperature. The limited reduction for the dark current of GaAs-based In0.83Ga0.17As photodetectors on the two-step temperature grown InxAl1−xAs buffer layers was ascribed to the contribution of impurities caused by the low growth temperature of InAlAs buffers.

TEM study of dislocations structure in In0.82Ga0.18As/InP heterostructure with InGaAs as buffer layer

In order to improve the quality of detector, InxGa1−xAs (x=0.82) buffer layer has been introduced in In0.82Ga0.18As/InP heterostructure. Dislocation behavior of the multilayer is analyzed through plane and cross section [110] by transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM). The dislocations are effectively suppressed in InxGa1−xAs (x=0.82) buffer layer, and the density of dislocations in epilayer is reduced obviously. No lattice mismatch between buffer layer and epilayer results in no misfit dislocation (MD). The threading dislocations (TDs) are directly related to the multiplication of the MDs in buffer layer.

Comparative research on reflection-mode GaAs photocathode with graded AlxGa1−xAs buffer layer

The graded Al compositional AlxGa1−xAs buffer layer can not only form continuous internal electric field from buffer layer to active layer but also optimize the interface properties by decreasing the misfit dislocations and stacking faults arising from lattice mismatch. By measuring the spectral response current (SRC) for two reflection-mode (r-mode) designed samples of graded and stationary Al compositional structure, we can find the special phenomenon that the graded structure had quite influence at the middle wavelength band from 550nm to 850nm, but not the short wavelength band from 400nm to 550nm, though the buffer layer can only absorb photon energy at the short wavelength band. Through the comparative research for designed samples through SPV before Cs–O activation and SRC after Cs–O activation, the graded structure can well optimize the key parameters such as LD, Ln, Sv and P. For the photon absorption lengths are relative little at the short wavelength band and relative long at the middle wavelength band, so the optimizations of key parameters have little influence on photo-excited electrons at the short wavelength band which are mainly excited from the region in active layer near surface barriers. The optimizations of key parameter, mainly the back interface recombination velocity (Sv), can have quite impact on photo-excited electrons at the middle short wavelength band which are mainly excited from the internal active layer near the back interface. This comparative research can help to well study the photo-emission theory and structure design on graded Al compositional design for r-mode GaAs photocathodes in the future research.

Comparative research for GaAs photocathodes with graded compositional AlxGa1−xAs buffer layer

The graded Al compositional structure design in AlxGa1−xAs buffer layer can well increase the quantum efficiency than the stationary Al compositional structure design, especially at short-wave band. For deep study, the samples of two structures were prepared by metal organic chemical vapor deposition for carrying out the comparative research between surface photovoltage before Cs–O activation and spectral response current after Cs–O activation. Through fitting calculations and analyzes, the mixed structure of graded Al composition in AlxGa1−xAs buffer layer and exponential doping concentration in GaAs active layer cannot only form continuous internal electric field throughout window layer and active layer, but also decrease the misfit dislocations and stacking faults arising from lattice mismatch at the AlxGa1−xAs/GaAs interface. The better interface properties can further decrease the back interface recombination velocity than the standard structure design. Furthermore the continuous internal electric field can well increase the electron diffusion and drift length for active layer and surface escape probability for surface barriers and can also drive more photo-excited electrons at short-wave band in buffer layer to cathode surface.

Study of the Growth and Dislocation Blocking Mechanisms in InxGa1−xAs Buffer Layer for Growing High-Quality In0.5Ga0.5P, In0.3Ga0.7As, and In0.52Ga0.48As on Misoriented GaAs Substrate for Inverted Metamorphic Multijunction Solar Cell Application

AbstractEffects of growth conditions and buffer structures on crystal quality of 1.9-eV In

Design of InxGa1−xAs buffer layers for epitaxial growth of high-quality In0.3Ga0.7As films on GaAs substrates

A phase field model is developed to simulate In0.3Ga0.7As thin films grown on an GaAs substrate with different buffer layer structures. Using this newly developed phase field model, an optimal step graded InxGa1−xAs buffer layer structure with four sub-layers of x = 0.09, 0.18, 0.27, and 0.33 is then proposed for epitaxial growth of high-quality In0.3Ga0.7As film on GaAs substrate. The strain distribution analysis by using the phase field model reveals that the compressive strain in this optimal heterostructure is partially balanced by the tensile strain caused by the uppermost two layers of In0.3Ga0.7As grown on top of In0.33Ga0.67As, which results in high-quality In0.3Ga0.7As film. The subsequent epitaxial growth of In0.3Ga0.7As films on GaAs substrates demonstrates that the surface RMS roughness and the full width at half maximum of X-ray rocking curve of as-grown In0.3Ga0.7As film with the optimal buffer layer structure are as low as 0.56 nm and 116′′, respectively, indicating very high film quality. These experimental results confirm the high effectiveness of the proposed approach to design buffer layer structure using our newly developed phase field model. This phase field model should be able to extend to hetero-epitaxial growth of other material systems apart from InGaAs.

Role of AlxGa1−xAs buffer layer in heterogeneous integration of GaAs/Ge

The material and optical properties of the GaAs/AlxGa1−xAs/Ge structures grown by metalorganic chemical vapor deposition were examined and found to be dependent of the Al content x. SIMS and PL measurements show that the 10 nm AlxGa1−xAs buffer layer with x = 0.3 and 0.6 are equally effective in suppressing the outdiffusion of Ge, whereas x = 1.0 gives the most abrupt interface. The best morphology with surface rms of 0.3 nm is obtained in the structure with x = 0.3 buffer layer. Analysis on change of strain in the AlxGa1−xAs buffer layer suggests that the compressive strain at the AlxGa1−xAs-GaAs interface is compensated by the tensile strain at the Ge-AlxGa1−xAs interface when x ∼ 0.3. AlxGa1−xAs lattice matched to Ge is crucial for better result in surface morphology, but higher Al content is preferred for eliminating the interdiffusion of atoms at the heterointerface.

Strong enhancement of direct transition photoluminescence with highly tensile-strained Ge grown by molecular beam epitaxy

Highly tensile-strained layers of Ge were grown via molecular beam epitaxy using step-graded InxGa1−xAs buffer layers on (100) GaAs. These layers have biaxial tensile-strain of up to 2.33%, have surface roughness of <1.1 nm, and are of high quality as seen with transmission electron microscopy. Low-temperature photoluminescence (PL) suggests the existence of direct-bandgap Ge when the strain is greater than 1.7%, and we see a greater than 100× increase in the PL intensity of the direct transition with 2.33% tensile-strain over the unstrained case. These results show promise for the use of tensile-strained Ge in optoelectronics monolithically integrated on Si.

X-ray diffraction analysis of step-graded InxGa1−xAs buffer layers grown by molecular beam epitaxy

High resolution X-ray diffraction reciprocal space mapping (RSM) is used to study the crystal quality of step-graded InxGa1−xAs buffer layers grown on GaAs (001) substrates by molecular beam epitaxy (MBE) using two growth methods. The lateral correlation length of the buffer layers are described by the FWHM of diffraction peaks along the lateral directions extracted from RSM, which are good indicators of the layer's crystalline quality. The quality improvement of InxGa1−xAs buffer layers grown at low temperature with in-situ annealing could be determined by XRD-RSM, which is also consistent with TEM results.

Tilt generation in step-graded InxGa1−xAs metamorphic pseudosubstrate on a singular GaAs substrate using a low-temperature grown InGaP interlayer

Metamorphic pseudosubstrates of In0.42Ga0.58As were grown by molecular beam epitaxy using step-graded InxGa1−xAs buffer layers grown either directly on a (001) GaAs substrate or on a GaAs substrate overgrown with a layer of low-temperature grown In0.51Ga0.49P (LT-InGaP). The structures were examined using x-ray reciprocal space mapping to determine the characteristics of the pseudosubstrates and buffer layers. For the sample grown on the LT-InGaP layer, the pseudosubstrate exhibited an asymmetric tilt around [11¯0] toward the [110] direction. Weak-beam dark-field electron imaging shows an imbalance of misfit dislocations with opposite sign Burgers vector. An explanation for this tilt is given and it is suggested that it may be responsible for the improved quality of epitaxial layers grown on such pseudosubstrates.

Characterization of metamorphic InxAl1−xAs∕GaAs buffer layers using reciprocal space mapping

The depth profiles of metamorphic InxAl1−xAs (0.05&amp;lt;x&amp;lt;1) buffer layers grown on GaAs substrates were characterized using the x-ray reciprocal space mapping. Three types of metamorphic samples were investigated and compared: step grade, single-slope linear grade, and dual-slope linear grade. The lattice mismatch, residual strain, crystallographic tilt, tilt azimuth, and the full width at half maximum were obtained from the reciprocal space maps. The tilt angle of linearly graded buffer layers stayed low at low In compositions until In≈60%, at which composition the tilt angle increased abruptly. All linear-grade samples had an untilted relaxed structure in the low In region (below 60% In) and a tilted structure in the upper, high In region (above 60% In). The average lattice mismatch between the untilted relaxed structure and the tilted structure determines the tilt angle. The tilt angle of the step-graded layers increased at a near-linear rate as the In composition was increased. The tilt azimuth was intermediate between the ⟨100⟩ and ⟨110⟩ in-plane directions. The x-ray full width at half maximum generally increased with the In composition, but tended lower toward surface. We suggest a possible design strategy for the linear-grade metamorphic buffer layer based on our result.

Novel direct MOCVD growth of InxGa1−xAs and InP metamorphic layers on GaAs substrates

A review of the technique of direct growing high-quality InxGa1−xAs or InP buffer layers on GaAs substrates by metal-organic chemical vapor deposition (MOCVD) is given. This low-temperature growth method benefits the improvement of metamorphic device performance. In this work, a simple and novel method of directly deposited thin InxGa1−xAs or InP buffer layers (< 1 μm) on GaAs substrates is presented, instead of strained-layer superlattice, two-step, graded or CS (compliant substrates) methods, while maintaining low dislocations, high crystal quality, and uniform and mirror like surfaces. For the direct growth technique of InxGa1−xAs on GaAs, we found an excellent quality In0.54Ga0.46As buffer of rms surface roughness of only 0.686 nm by AFM and FWHM of 925 arcsec by XRD can be obtained at a low growth temperature of 440∘C with a constant Ga/Ingas partial pressure of 5. The superior results are mainly due to the use of low temperature growth technology. In addition, we also found this growth technology is available to directly grow InP buffer layers on GaAs. Experimental results conclude that the growth temperature of 480∘C in harmony with the V/III ratios range of 130–210 is a suitable window to directly grow InP on GaAs substrates.

Study of the crystal quality and Ga-segregation in ZnSe films grown by molecular beam epitaxy on Al xGa 1− xAs and In xGa 1− xAs buffer layers on GaAs substrates

In the present work, we report a study of the molecular beam epitaxial growth of ZnSe on GaAs substrates using Al x Ga 1− x As and In x Ga 1− x As ternary alloys as buffer layers. When growing ZnSe directly on a thermally desorbed GaAs substrate, surface segregation of Ga across the film towards the ZnSe surface was observed by secondary ion mass spectroscopy. We demonstrate that the use of AlGaAs buffer layers is very effective to suppress the Ga surface segregation. The characterization of the films by reflection high-energy electron diffraction, atomic force microscopy, transmission electron microscopy, photoluminescence and photoreflectance spectroscopy revealed that the best crystal quality ZnSe films were obtained for buffer layers with In or Al concentrations of 1%.

Characterization of ZnSe films grown on GaAs substrates with In xGa l− xAs and Al xGa 1− xAs buffer layers

In this work we present a study of ZnSe films grown on buffer layers of the ternary compounds In x Ga 1− x As and Al 1− x Ga x As by means of photoreflectance (PR), and photoluminescence (PL) spectroscopies and transmission electron microscopy (TEM). The buffer layers of the ternary compounds were grown by molecular beam epitaxy (MBE) employing concentrations with 0.01< x<0.3 and thickness of 5000 Å. The films of ZnSe were grown by MBE in a separate II–VI system with a thickness of 6000 Å. The measurements of PR allowed us to determine the energy value of the ZnSe band gap, the magnitude of the internal electric fields, and to evaluate the quality of the buffer layers and the ZnSe film. The intensity and width of the excitonic related peaks in the PL spectra and the presence of lines related with defects allowed us to assess the crystalline quality of the films. TEM results have a good correlation with the optical spectroscopies showing that the better samples are those grown on buffer layers of Al 0.01Ga 0.99As and In 0.01Ga 0.99As.

Photocapacitance study of bulk deep levels in ZnSe grown by molecular-beam epitaxy

The influence of initial growth conditions and lattice matching on the deep level spectrum of n-ZnSe grown on GaAs by molecular-beam epitaxy is investigated by means of deep level optical spectroscopy. A detailed study of both the steady-state and transient photocapacitance allows us to measure optical threshold energies, concentrations, and emission rates of electronically active defects in the ZnSe layer. Several deep levels are found in the ZnSe layer at Ec−Et=1.15, 1.46, 1.90, and 2.25 eV with concentrations in the 1012–1014 cm−3 range. When a 2-nm-thick composition controlled interface layer is grown at different beam pressure ratios prior to the ZnSe growth, a distinct decrease in the 1.46 eV level concentration with increasing Se content is found. Deposition of a lattice-matched InxGa1−xAs buffer layer prior to the ZnSe growth reduces the concentration of both the 1.15 and 1.46 eV levels by over an order of magnitude, indicating the role of lattice matching in the ZnSe overlayer. We also perform depth profiling of the defect distributions within the ZnSe overlayer to see the effect of the ZnSe thickness on the concentration of these levels as well as their possible association to the ZnSe/GaAs interface. We find that only the 2.25 eV level concentration shows a dependence on depth, increasing as the II–VI/III–V interface is approached.

Zn 0.85 Cd 0.15 Se active layers on graded-composition InxGa1−xAs buffer layers

We investigated the structural and optical properties of Zn0.85Cd0.15Se epilayers for blue optical emission on lattice-matched InxGa1−xAs buffer layers. Both the II–VI layers and the III–V buffers were grown by molecular beam epitaxy on GaAs(001) wafers. A parabolic In concentration profile within the graded-composition InxGa1−xAs buffers was selected to control strain relaxation and minimize the concentration of threading dislocations. Dislocation-free II–VI growth was readily achieved on the graded buffers, with a Rutherford backscattering yield ratio reduced by a factor of 3 and a deep-level emission intensity reduced by over two orders of magnitude relative to those observed following direct II–VI growth on GaAs. The surface morphology of the materials, however, was found to replicate the crosshatched pattern of the underlying InxGa1−xAs substrates.